The present invention relates to machine rolls for making low basis weight webs, and more specifically to a roll heated with hot oil for use in the manufacture of cellulose webs or polymeric webs or films.
One of the problem areas in providing a hot-oil heated roll, such as an embossing roll, is that with all currently available systems of which applicant is aware, localized hot or cold spots develop on the surface of the roll due to undesirable flow patterns and variable heat transfer from the hot oil, which results in nonuniform bonding patterns within the cellulose or polymeric web. One desirable method of distributing heat to the surface of the roll is by the use of an annular space between an outer and an inner roll, with the hot oil being distributed throughout this annular space between the rolls.
The ultimate purpose of using a heated fluid, such as oil, is to approach a uniform temperature over a predetermined active width of the surface of the roll where embossing or calendering is to take place. Nearly uniform temperature is important not only for proper thermal conditions in embossing or calendering, but also to maintain a controlled roll profile and resulting pressure distribution across the roll width in the nip between the rolls. Local hot spots in a roll causes local expansion and result in areas of excessive nip pressure. A nonuniform temperature, particularly from the ends of the roll to the center, alters the fit of each roll to its counterpart and areas of undesirably higher or lower compacting pressure result.
A heated roll as envisioned herein, such as an embossing roll, typically has a somewhat greater width than the width of the web to be produced so that end conditions in the roll have minimal effect on the active length of the roll nip. However, variation in the level of turbulence has been found to produce differences in heat transfer from the heating medium to the roll. Therefore, the result is nonuniform temperature in many prior art devices where the heating medium is introduced at one end of the roll so that it may move axially to the opposite end. Other prior art suggestions include introducing the oil at regularly spaced intervals over the entire width of the roll causing local areas of high turbulence. In any case, the heated oil is introduced perpendicular to the inner surface of the roll to be heated. This results in a "hot spot" in the area where the oil impinges on the inner surface of the roll with "cold spots" therebetween.
Another problem with prior art devices is that the flow rates at various points from oil entrance to oil exit are nonuniform. Oil introduced at one end of the roll tends to speed up as it moves axially toward the oil exit when oil is introduced at spaced axial intervals along the roll, since the flow rate of the total oil flow is equal to the sum of the flows from each oil inlet port.
One prior art roll-heating method is shown in U.S. Pat. No. 3,838,734, Killmartin. Although this device is intended to supply a coolant to a chill roll, it has been used as a basis for heated rolls. In this apparatus, coolant is delivered to an inner chamber within an inner shell, the inner shell being provided with nonuniform-diameter axially spaced openings from end to end constructed so that the flow rate is equalized through holes at different axial locations. Fluid is conducted axially through an annular space between the inner and outer shells in order to cool the outer surface of the outer shell.
U.S. Pat. No. 4,050,510, Theysohn illustrates a roll heating configuration wherein an outer shell has a plurality of axial passages drilled therein beneath the outer surface of the shell. An end structure is provided with a plurality of radial passageways communicating with the axial passages so that steam supplied to the radial passageways will heat the roll surface through the axial passages.
While these devices perform their intended function, that is, heating or cooling the outer roll surface, they are not capable of doing so in a totally acceptable manner. When the rolls are used in conjunction with relatively thick cellulosic or other types of webs, precision in the temperature profile across the roll width may not be necessary. However, when dealing with extremely thin or low basis weight webs, such as spunbonded or meltblown nonwoven films or webs, or tissue grade cellulose webs, it is critical that the temperature be uniform across the width in order to assure that the rolls seat properly to produce a web with uniform embossing or calendering across the roll width.